This invention relates to an electroacoustic apparatus and, more particularly, to a loudspeaker apparatus for reproduction of low frequencies in the audible range. Specifically, this invention relates at a low frequency folded exponential horn loudspeaker apparatus which has a bifurcated sound path for direction of sound waves from at least one electroacoustic transducer to a volume into which the sound waves are radiated.
High fidelity sound reproduction requires reproduction of low frequencies in the audible range. W. B. Snow, "Audible Frequency Ranges of Music, Speech, and Noise", Jour. Acous. Soc. Am., Vol. 3, July, 1931, p. 155, for example, indicates that high fidelity sound reproduction of orchestral music requries that the frequency band should extend to as low as 40 Hz.
It is well established that loudspeakers, in order to reproduce a given frequency range, must have physical dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn type of loudspeaker, for example, the area of the exponential horn mouth is determined based on the wavelength of the lowest frequency in the range to be reproduced.
At an early date, to obtain high fidelity sound reproduction with exponential horn loudspeakers, and, in particular, the inclusion of low frequencies in the audible range, large straight axis exponential horn loudspeakers were constructed. For example, theater loudspeakers as large or larger than eight feet in length and four feet by four feet in transverse physical dimensions were built in order to obtain reproduction of low frequencies in the audible range. More recently, the physical dimensions of exponential horn loudspeakers have been reduced by folding the exponential horn. See, for example, H. F. Olson, Elements of Acoustical Engineering, 1947, pp. 206-209; P. W. Klipsch, "La Scala", Audio Engineering Society Preprint No. 372, April, 1965; H. F. Olson and F. Massa, Jour. Acous. Soc. Am., Vol. 8, No. 1, 1936, pp. 48-52; E. C. Wente and A. L. Thuras, Jour. A. I. E. E., Vol. 53, No. 1, 1934, pp. 17-24; J. K. Hilliard, Tech. Bul. Acad. Res. Conv., March, 1936, pp. 1-15.
Prior art low frequency folded exponential horn loudspeakers, such as those which are disclosed in the above-cited references, are, nevertheless, bulky and structurally complex due to the structure of the folded exponential horn which defines the sound path from the electroacoustic transducer to the volume into which sound waves are radiated. In this category are the low frequency loudspeakers with folded exponential horns that are divided to provide bifurcated sound paths as disclosed in the above-cited H. F. Olson, P. W. Klipsch and J. K. Hilliard references.
Specifically, each of these references discloses a structurally complex low frequency loudspeaker that includes a complicated baffle arrangement to form the folded exponential horn which defines the bifurcated sound path. Each of the H. F. Olson and J. K. Hilliard references discloses a doubly folded exponential horn which defines the bifurcated sound path so as to first direct sound waves toward the front, then toward the rear and then again toward the front of the low frequency loudspeaker. Consequently, the bifurcated sound path is a complex serpentine sound path. Each of the J. K. Hilliard and P. W. Klipsch references discloses a folded exponential horn which defines the bifurcated sound path wherein the folded exponential horn includes a pyramid- or cone-shape baffle that must occupy a precise position with respect to the throat so as to make construction difficult. Moreover, the P. W. Klipsch reference discloses a structurally complex low frequency loudspeaker that includes a centrally located electroacoustic transducer and back air chamber arrangement which requires precise orientation since the back air chamber walls also form part of the folded exponential horn which defines the bifurcated sound path.
Although the low frequency loudspeakers that are disclosed in these references afford acceptable sound reproduction characteristics at low frequencies in the audible range, the complex structure has necessitated considerable craftsmanship in construction and close attention to quality control and has resulted in high cost. Furthermore, the physical dimensions and weight of these low frequency loudspeakers due to structural complexity and amount of materials that is incorporated to construct the baffle arrangement has often resulted in a size too large to fit through a standard doorway and too weighty to manually carry.
One objective of this invention is to provide a low frequency loudspeaker apparatus of the folded exponential horn type which does not require the cooperation of independent boundary surfaces adjacent to the mouth to reproduce low frequencies in the audible range.
Another objective of this invention is to provide a low frequency folded exponential horn loudspeaker apparatus for the performing arts for high fidelity sound reproduction.
A further objective of this invention is to provide a low frequency folded exponential horn loudspeaker apparatus capable of generating 20-30 acoustic watts continuous average power.
Another objective of this invention is to provide a low frequency folded exponential horn loudspeaker apparatus which operates at optimum efficiency.
An additional objective of this invention is to provide a loudspeaker apparatus which has a smooth amplitude response over the lower range of audible frequencies that is necessary for high fidelity sound reproduction.
Another objective of this invention is to provide a low frequency folded exponential horn loudspeaker apparatus with a minimum of amplitude and frequency modulation distortion.
A further objective of this invention is to provide a low frequency folded exponential horn loudspeaker apparatus with a simplified structure without sacrificing high fidelity sound reproduction.
Another objective of this invention is to restrict one physical dimension to a maximum of 32 inches to allow passage of the low frequency folded exponential horn loudspeaker apparatus through a standard doorway without sacrificing response at low frequencies in the audible range.